Interfacial Effect On High Frequency Properties And Perpendicular Magnetic Anisotropy Of Fe-based Soft Magnetic Films

With the rapid development of electronic devices,the electromagnetic component are highly miniaturized and thinned.In addition,high storage density and high frequency microwave response are also required.Soft magnetic materials are widely used in sensors,converters,memories,etc.,due to their low coercivity,high magnetic permeability,low loss power,easy magnetization and demagnetization properties.Therefore,the research on Fe-based soft magnetic films and ultra-thin films has become a hotspot gradually.In this thesis,the roughness effects of the Si substrate and intermediate layer Al on the static and dynamic magnetic properties of CoFeB and FeNi ferromagnetic films were investigated.The relationship between coercivity,damping,resonance frequency of theses soft magnetic films and roughness was characterized by VSM and FMR tests.At the same time,the perpendicular magnetic anisotropy of the FeCo/Pt multi-layer film was generated by the interface effect,which can be controlled by annealing treatment,changing the seed layer and the thickness of the ferromagnetic layer as well as the number of periods in the multilayer films.The magneto-optical Kerr microscope was used to characterize the reversal and motion of the domain walls in the sample.The sample can serve as a free layer in the Magnetic Tunnel Junctions(MTJs)structure,making it more efficient in information storage of Magnetic Random Access Memory(MRAM)devices.The main results are as follows:(1)The Si substrate surface of the CoFeB thin film was etched by using different concentrations of NaOH solution to change its roughness,and the sample was measured by VSM and FMR to characterize its static and dynamic magnetic properties.The experimental results are as follows:with the gradual increase of NaOH concentration,the surface roughness of Si substrate increases first and then decreases.The variation of coercivity H_c and effective damping?is monotonous with the change of substrate roughness.Similarly,in the FeNi thin film,by sputtering different thicknesses of Al as the intermediate layer of the FeNi soft magnetic film,the roughness increases first and then keeps a stable trend as the thickness of the Al increases.Meanwhile,the variation of H_c and?is also monotonous with the change of intermediate layer roughness.The results show that the change of roughness can effectively influence the coercivity and effective damping of the film.(2)Firstly,when the thickness of FeCo layer is between 0.4 and 0.8 nm,the FeCo/Pt multi-layer films display a good perpendicular anisotropy,otherwise,the samples exhibit an in-plane anisotropy.The nucleation,growth,expansion,reversal of magnetic domain and domain wall motion process are studied by magneto-optic Kerr microscope.Secondly,by changing the seed layers and their thickness in the samples,it is found that the generation of perpendicular anisotropy is ascribed by the introduction of the(111)texture of the Au and Pt seed layer.Lastly,FeCo/Pt multi-period films annealed at different temperature by vacuum magnetic field heat treatment are studied,it is found that,after annealing at an appropriate temperature,the perpendicular anisotropy is enhanced.However,when the annealing temperature reaches 400°C,which will weaken the perpendicular anisotropy of the film.